the easiest way I've found to measure head clearance is to use a very tiny piece of modeling clay- just as with full size engines.

With the head removed, lay a tiny piece of clay on the top of the piston near the cylinder wall. Assemble the engine and roll it over only once.

Remove the head and carefully remove the clay from the piston. I've found if you put oil on the head button, this will be very easy as it will not stick to the head.

Now cut the clay in half with a razor blade, and measure the thickness with a micrometer or calipers.

I know solder is relatively soft, but clay surely will not damage anything in your delicate engine.